Signaling system



arch 18, 193%. R. K. POTTER 1,750,688

SIGNALING SYSTEM Filed Nov. 5, 1927 Oscillator IOQOG.

INVENTOR. BEPQM A TTORNEYS.

Patented Mar. 18 1930' RALPH K. POTTER, OF NEW YORK, N. Y., ASSIGNOR '10 AMERICAN TELEPHONE AND TELEGRAPH COMIPANY, A CORPORATION OF NEW YORK SIGNALING sYsrEm' Application filed November 8, 1927. Serial No. 230,908.

This invention relates to signaling systems, and particularly to systems for the secret transmission of waves.

The invention is particularly applicable to the transmission and reception, with secrecy, of waves, such as signals and the like, lying within a considerable band of frequencies, as in the case of speech..

One method of obtaining privacy in the transmission and reception of waves by radio depends upon the division of a band of frequencies, as in the case of speech, into sub,- bands, and the rearrangement of these subbands before transmission. At' the receiv- 5 ing end of such a system, the sub-bands may original signal may be reconstructed from the component sub-bands.

If highly selective filters are emplo ed to divide the band of signals into subands,

the edges of the sub-bands, i. e., frequencies near the cut-off points of the filters, are subjected to considerable undesirable distortionso that upon selection of the varioussubbands at the receiving terminal of the system, unrelated and extraneous frequencies may be introduced from one sub-band to the other.

In order'to overcome the loss in intelligibility caused by the suppression of component frequencies and the introduction of unrelated and extraneous frequencies at the edges of the sub-h ands, it is proposed, in this invention, to divide the band of signals into a plurality of successive, overlapping sub-bands and to separately modulate the various sub-bands upon spaced carrier freuencies so that side bands of the carrier requencies corresponding to the various subbands may be transmitted through space to a distant receiving station, separated by a small frequency interval. At the receiving terminal of such a system, it is proposed to select the various received side hands into appropriate circuits and to bring these side bands back to their original frequency relations by subjecting these side bands to demodulation employing similar carrier waves spaced at substantially the same intervals as the carrier waves are spaced in modulation at the transmitting station.

Further objects and features of this inve tion will be more fully apparent from the currents resulting from speaking into a microphone S may be applied to an amplifier having any number of stages of amplification and of any well-known type, preferably of the vacuum tube type, as shown. These speech currents are applied through a transformer T to the vacuum tube- V of the first stage of amplification. The amplified energy is then applied through another transformer T to another stage of amplification having a vacuum tube V After being further amplified, these speech currents are then applied through the transformer T to a plurality of sub-dividing electrical wave filters which may be employed to sub-divide the speech band into a plurality of successive overlapping sub-hands. Only two of these sub-dividing electrical wave filters F and F are shown herein merely for the purpose of illustration. These electrical wave filters are preferably so designed as to pass overlapping portions of the speech frequency range. For example, filter F may selectively transmit frequencies from 200 to 1600 cycles and filter F 2 may selectively transmit frequencies from 1500 to 2600 cycles, it being assumed that the essential speech frequencies are to be found within the range extending from 200 to 2600 cycles. This result may be accomplished by constructing each of these filters in the form of the well-known band filter composed of a plurality of sections, as disclosed in the patent to G. A. Campbell, N0. 1,227;113, dated May 22, 1917. It will be obvious that any number of filters may be used as desired and that. only two of these are shown herein for illustrative purposes and that the band widths of the respective filters may be suitably chosen to-correspond with the number of filters used. It will be obvious that the filters shown herein are employed for the purpose of sub-dividing. the speech frequency band into a plurality of successive overlapping sub-bands, each of a relatively narrow frequency range. Each of these sub-bands, if received, may, of itself, normally be incapable of providing intelligible signals;

With the arrangement of two filters shown in the drawing, it will be seen that the lower sub-band may be transmitted by the filter F and that the higher sub-band beginning at a fre uency below the upper limit of the lower su -band may be transmitted by the filter F Obviously, these filters transmit sub-bands which, if properly combined, represent a band of signals corresponding to the speech signals originating in the microphone S: k

The sub-bands transmitted by the filters F and F may then be applied to amplifiers 1 and A respectively. These amplifiers may be of any well-known type, preferably of a vacuum tube type. Clearly, these amplifiers are employed to separately amplify-the respective channels.

The sub-bands amplified by the amplifiers A and A are then applied through transformers T and T to corresponding duplex modulators having three-element vacuum tubes M and M and M. and M respective ly. The transformers T and T are employed to impress the sub-bands amplifiedby amplifiers A and A iip'on the input circuits of the vacuum tubes of the corresponding duplex modulators, these vacuum tubes besub-bands transmitted'bythe filters in their i'ng inpush-pull relationship. Oscillators U and O transmit currents of carrier waves through transformers T and T to the branches common to the input circuits of the modulator tubes M and M and M and M respectively. The output circuits-of these modulator tubes are in parallel relationship with respect to common plate batteries, these output circuits being connected through the primary windings of the transformers T and T respectively. These modulators are of the carrier suppression typeas disclosed in patent to J. R. Carson, No. 1,343,306, dated June 15, 1920. The oscillators O and 0 may, of course, be of anyiwell-known type, preferably of the vacuum tube type, such as is shown in a patent to R. V. L. Hartley, No. 1,356,763, dated October 26, 1920. Inasmuch as each of the duplex modulators, such as the one having vacuum tubes M and M is supplied by a wave of a carrier frequency by means of-an oscillator such as the oscillator- C1 and is also supplied with a sub-band such as is transmitted by the filter F and the amplifier A there will be'produced for transmission two side bands corresponding to each sub-band, as is well known in the art. The outputs of the modulators having the vacuum tubes M and M and M and M are then applied to filters F and F through transformers T and T respectively Filters F andF may, if desired be of thetype described in the aforementioned Campbell patent, each freely transmitting one of the side bands resulting from modulation corresponding to a' 'sub-band, while substantially suppressing the otherrside band corresponding to the same sub-band as. well as all other inter-modulation components; Thus, these filters may,.if desired,-be designedso as to suppress the. lower side bands and freely transmit the upper side bands, these upper side bands being obviously of widths substantially equal-tothe widths of thesubbands freely transmitted by the filters and amplifiers in the corresponding transmission paths, though raised in the frequency specthen applied through a transformer T to another duplex modulator having the vacuum tubes M and M these vacuum tubes being also in push-pull relationship. An oscillator 0 transmits current of another and preferably higher frequency through a transformer T to the branch common to the input circuits of the vacuum tubes M and M The output circuits of these vacuum tubes are also in parallel relationship with respect'to a commonplate battery. these output circuits being connected through the primary of the transformer T This duplexmodulator is, ofcourse, of the type disclosed in the aforementioned Carson pat'ent and the oscillator O may be of any well-known type, prefer side-bands corresponding to two sub-bands,

there will be produced for transmission four side bands of the wave emitted by the oscillator 0 corresponding to the two side bands transmitted thereto and impressed upon the input circuits of the vacuum tubes i 5 and M the latter side bands corresponding to the two sub-bands. All four side bands are then transmitted to a filter F through a transformer T This filter maybe of the general type disclosed in the aforementioned Campbell patent and is employed primarily to freely transmit two of the side bands impressed thereon by the duplex modulator, while substantially suppressing the other two side bands as well as all intermodulation comthrough a transformer T to an amplifier V preferably of the vacuum tube type, as shown. Amplifier V is employed for the purpose of amplifying the side bands of the wave of the oscillator 0 freely transmitted through transformer T by thefilter F The amplified energy is then impressed upon a trans-- mitting antenna circuit, comprising a transmitting antenna TA, a condenser C and the secondary winding of a transformer T' Thus, there may be transmitted through space a plurality of side bands of a high frequency wave, one corresponding to each of the subbands resulting from one of the portions into which the original speech bands were subdivided.

One form of receiving station based upon the principles of this invention for reproducing intelligible speech from the side bands transmitted from the station of Fig. '1 is illustrated in Fig. 2. The side bands of the car rier or radio wave transmitted'thro ugh space from the station of Fig. 1 are received on the receiving antenna RA which is connected in the antenna circuit along with a condenser C and-the primary winding of a transformer T A transformer T is employed to impress the receit'cd side bands upon an amplifier, preferablyof the vacuum tube type. comprising a three-element vacuum tube V as shown. The received side bands are thus amplified and then transmitted through a transformer T to filters F and F Thesefilters are employed forselectively transmitting the side bands received by the antenna circuit of Fig. 2. Thus, filter F may transmit a side band which corresponds to one of the subbands into which the speech signals originating in the microphone S at the transmitting station of Fig. 1 were divided and filter F may transmit the side band which corresponds to the other of the sub-bands into which the speech signals originating in this microphone S .were'divided. It will be obvious that filters F and F may, if desired, be of the aforementioned Campbell type, each freely transmitting one of the side bands corresponding to one of the sub-bands above mentioned, while substantially suppressing all of the other frequencies.

The side bands selected by filters F and F 7 are then transmitted through transformers T and T to demodulators or detectors having the vacuum tubes D and D respectively, each of the vacuum tube type. Oscillators O and O transmit currents of spaced frequencies through transformers T and T5,, to these detectors or demodulators, respectively. These oscillators may be of any well known type, preferably of the vacuum tube type such as is described in the aforementioned Hartley patent. These oscillators may be spaced in frequency at substantially the same interval that the frequencies of the currents of oscillators O and 0 are spaced at the transmitting station of Fig. 1. Moreover, the frequencies of these oscillators are sufficiently high so that, as a result of the action based on thewell-known principles of homodyne detection, low frequency waves will be transmitted to the output circuits of these detectors which contain components corresponding to those which were impressed upon the balanced, duplex modulators having the vacuum tubes M and M and M and M ,.respectively, at the transmitting station of Fig. 1. These low frequency waves are transmit ted through transformers T and T to a receiving device such as a telephone receiver R. These low frequency waves are thus combined in order to reproduce the signals generated in the microphone S at the transmitting station.

The principles of this invention have been shown as applied to a system involving the sub-division of speech signals into two subbands merely for the purpose of illustration. If it be assumed that transmission is to take place involving a 3000-cycle voice band extending from 100 cycles to 3100 cycles, four sub-bands may, for example, be employed. Four sub-dividing filters may then be used to sub-divide the speech frequencies into successive overlapping sub-bands. These sub- .bands may, for example. extend from 100 to 950 cycles; from 750 to 1700 cycles; from 1500 to 2450 cycles; and from 2250 to 3100 cycles. These sub-bands have widths of 850, 950, 950 and 850 cycles, respectively. If these subhands he superimposed upon carrier waves having frequencies of. for example, 40,000, 40,300, 40,600 and 40,900 cycles, respectively, then by suitable modulation and consequent suppression of the unmodulated carrier wave and one of the side bands resulting from m'odhigh frequency waves spaced at substantially the same interval as are the waves employed in modulation at the transmitting station, in

order to reproduce the signals corresponding to the sub-divided components of the trans.- mitting station. In the latter illustration, it will be obvious that the speech signals are sub-divided into successive, overlapping subbands and that, as a resultsfof; modulation upon suitably spaced carrier frequencies, the

sub-bands are. raised in the frequency spectrum, spaced so as not to overlap. .At the receiving station, the spaced bands of fre: quencies are beaten with carrier waves of similarly spaced frequencies in order to reproduce the successive overlapping sub-bands 'such asare found at the transmitting sta-' bands and atthe same time afforded acertain amount of privacy. As is well knownin systems of singleside band reception, if the carrier wave is displaced from its normal position, the quality of the signal is rapidly depreciated and the intelligibility falls ofi' by a corresponding amount. From experiments, it has been" determined that at a displacement of, for example, 700 cycles, the intelligibility is less than 10%. From these experiments, it has also been determined that the spacing-of the side bands corresponding to sub-bands by 100 cycles might be expected to give an intelligibility of approximately 40%. If.'lI1 addition to employing the principles outlined hereinabove, the sub-"bands be inverted and then interchanged at the transmitting station, and if the inverse process be employed at the receiving station, it seems reasonable, to expect that considerable privacy could be obtained without seriousmutilation of the signal.

\Vhile this invention has been shown and described in one particular embodiment mere ly for the purpose of illustration, it is to be distinctly understood that the general prin-' ciples of this invention may be applied to other and' widely varied organizations with out departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. The method of'transmission of waves having a band of frequency components,

.which comprises dividing said band into a plurality of sub-bands which overlap each other, modulating. said sub-bands on a plurality of carrier waves of different frequencies, one corresponding to each sub-band,

simultaneously.

2. The method of transmission of speech waves comprising sub-dividing the speech Waves into a plurality of sub-bands of frequency components which overlap each other in the'frequen'cy spectrum, the components comprised in any one sub-band being incapable of reproducing understandable speech, modulating said sub-bands upon a plurality of carrier Waves of different frequencies, one corresponding to eachsub-band, so' as to produce a plurality of side bands of the respective carrier waves, one side band corresponding to each sub band and occupying the same space in the frequency spectrum as was occupied by the sub-band beforemodw lation, said side bands being spaced from' each other by definite frequency interva transmitting all ofsaid side .bandss taneously. r Y

. 3. The method ofspeech transmission which comprises sub-diivid ing speech waves into aplurality of overlapping sub-bands,

each of definite frequency limits, modulating saidsub-bands upon a plurality of carrier- Waves of different frequencie's,'one carrier wave corresponding t v each sub-band, suppressing the transmission of the unmodulated carrier component and one of the twoside bands resulting from modulation of the car- 'rier wave, whereby a plurality of side bands are produced spaced in the frequency spectrum, and transmitting all of said spaced side bands simultaneously.

4c. The method of wave transmission which comprises sub-dividing a band of signals into a plurality of successive overlapping sub bands, each of which occupies a definite space in the spectrum, modulating said sub-bands upon a plurality of carrier waves of difierent frequencies so as to produce a plurality of side bands ofthecarrier waves spaced at definite intervals in the spectrum, and modulating the resultant'side bands upon a radio wave of a very high frequency for transmission to a distant point.

5. The methodof receiving a plurality'of bands of signals spaced in the frequency spectrum which, if combined, overlap and representall of the frequency components of speech signals, which consists in beating said spaced bands of signals with-a plurality of carrier waves of'difierent'frequencies, one corresponding to each received band, and.

combining the resultant products so as to derive the overlapping bands in order to reproduce all of the. frequency components representingthe speech signals.

6. The method of signaling which consists memes 5 in sub-dividinga band of signals into a plurality of successive overlapping sub-bands, modulating said sub-bands upon a plurality of carrier waves of different frequencies, suppressing the unmodulated carrier components and one side band of each carrier wave resulting from modulation and freely transmitting the other side bands which are spaced at definite intervals in the frequency spectrum, receiving the transmitted side bands, beating said received side bands upon a plurality of waves which dilfer from each other in frequency as do the carrier waves employed in modulation, and combining the beaten products in order to reproduce the original signals.

7. The method of signaling which consists in sub-dividing a band of signals into a plurality of successive overlapping spb-bands, each of which occupies a definite space in the spectrum, and modulating said sub-hands upon a corresponding plurality of carrier waves of difierent frequencies so as to produce a plurality of side bands of the carrier waves spaced at definite intervals in the spectrum.

8. A- wave transmission system comprising means for producing a band of signals, means i for sub-dividing said band of signals into a plurality of successive overlapping subhands, each. of said sub-bands occupying a definite space in the spectrum, means for producinga plurality of carrier waves of dilfer 'ent frequencies, one of said carrier waves corresponding to each of said sub-bands, and means for modulating said sub-bands upon trum, means for reconverting said spaced sub-bands into the corresponding plurality October, 1927.

RALPH K. PGTTER.

the corresponding carrier waves so as to prov duce a plurality of side bands of the carrierwaves spaced at definite intervals in the speclating said sub-bands upon said carrier waves,

means for suppressing the unmodulated car rier component and one side band of each carrier Wave resulting from modulation and freely transmitting the other side bands which are spaced at definite intervals in the frequency spectrum, means for deriving from said spaced side bands a plurality of corresponding overlapping sub-bands substantially the same as those derived in sub-dividing said band of signals, and means for combining the resultant overlapping sub-bands in order to reproduce the original signals.

10. A signaling system comprising, in combination, means for producing a band of signals, means for deriving a plurality of overlapping sub-bands therefrom, means for spacing said sub-bands in the frequency spec- 

